THE SOVIET shootdown of Korean Air Lines Flight 007 on September 1, 1983, has joined the list of unsolved mysteries with an insatiable audience among publishers.

R.W. Johnson's Shootdown is at least the fourth English-language book on the subject; there have been numerous articles in learned and not-so-learned journals, and a book from reporter Seymour Hersh is due soon. The word-barrage will doubtless continue as long as it is impossible to say how the Boeing 747 jumbo jet came to be more than 300 miles off course, deep in Soviet territory. When it was shot down, all 269 people on board were killed.

Johnson's hypothesis, oversimplified, is that U.S. foreign policy hardliners led by CIA Director William Casey and National Security Advisor William Clark approved the dispatch of the Korean jet on its strange course to test, among other things, the new Soviet radar array at Krasnoyarsk. That radar is an alleged violation of the unratified SALT II treaty.

Flight 007 was to do nothing so overt as taking pictures, but as a "passive probe" it would trigger Soviet radar and surveillance devices so that U.S. satellites and other electronic intelligence collectors could read capabilities they rarely "see." The United States did not expect the plane to be shot down, the theory continues. When it was, U.S. officials covered their role with a massive anti-Soviet propaganda effort that included heavy doses of disinformation, all subscribed to by a know-nothing president who thinks of the Soviet Union as an evil empire.

There is little question that the U.S. government has stonewalled on the issue of why the plane wasn't warned that it was off course. It seems logical, considering what is known about U.S. intelligence-collecting capabilities, that somebody, somewhere in the U.S. government, knew as it was happening that Flight 007 had strayed, even though the plane was beyond the range of civilian air traffic control radar systems. The U.S. response is that information such as radio transmissions of the Soviet fighter pilots used in the vigorous anti-Soviet propaganda effort during the days following the shootdown was collected automatically on tapes, then recovered and translated.

In the absence of a more thorough U.S. explanation, the kind of hokum that is put forth in Johnson's book and several other efforts will never be dispelled. One does have to wonder just what the big national secret is that has to be protected from news organizations, lawyers and others who have sought additional data. That wondering is what feeds the U.S.-Is-Guilty group.

The problem with Johnson's book is not that he exploits this secretiveness on the part of the United States. The problem is that he discredits his thesis with disinformation of his own on points that are easily checked. The reader is put on alert on page 2, when Johnson quotes the International Civil Aviation Organization's report of the accident. ICAO ran several flight simulations with various misprogrammings of navigational computers and concluded that two scenarios would produce approximately the route KAL 007 flew. Others investigating the case have duplicated ICAO's work.

ICAO says that "each of the simulation scenarios assumes a considerable degree of lack of alertness and attentiveness on the part of the entire flight crew but not to a degree that is unknown in international civil aviation." Johnson limits the quote to the words "a considerable degree of lack of alertness and attentiveness on the part of the entire flight crew." Then Johnson tells us what a great crew Flight 007 had. The effect is to make impossible something that has happened many times, a misprogrammed computer guiding a carelessly monitored flight. Just that scenario is the generally accepted explanation among non-conspiracy theorists.

Johnson asserts that the National Transportation Safety Board was ordered off the case by the State Department. I know from my own reporting at the time of the accident and from rechecking since that that is garbage. Under an international treaty to which the United States, the Soviet Union, Japan and Korea are all signatories, responsibility for the investigation rested with either the Soviets or the Koreans, not the safety board, depending on whether the wreckage came down in Soviet or international waters. Safety board representatives participated as observers in the Korean investigation, as is customary, but the United States was shut out of the Soviet investigation. Some ICAO officials did visit the Soviet Union.

Johnson also finds highly suspicious the fact that Clark left the White House for the relative peace and quiet of the Interior Department almost immediately after the shootdown. Reporters covering the White House at the time know that the exhausted Clark had been looking for a way out long before the shootdown and that the opportunity presented itself when former Interior Secretary James Watt put his foot in his mouth once too often.

Johnson is a fellow in politics at Oxford University. His book does not replace Alexander Dallin's effort, Black Box, as the fairest, most accurate and by far most readable serious treatment to date of the mystery surrounding Flight 007. Douglas B. Feaver is a reporter on the national staff of The Washington Post.